Method of curing thermoset resin with visible light

ABSTRACT

A transparent mold system (TMS) allows rapid and economical production of thermoset fiber reinforced plastics (FRP) and other thermoset resin parts by heating with visible light energy without using an autoclave, photosensitive resins, or ultraviolet (U.V.) sensitive chemical curing agents. Molding times are accelerated to minutes instead of hours, and the molds are made of transparent plastics materials that can be economically produced by injection, casting, vacuum or pressure forming to replace metal molds and laid-up FRP molds.

RELATED APPLICATION

This application claims the benefit of the filing date of provisionalpatent application Ser. No. 60/000,506, filed Jun. 26, 1995.

BACKGROUND OF THE INVENTION

In the development of low cost organic matrix composite materials, ithas been determined that eliminating the need to process the materialsin an autoclave will provide for a significant cost reduction. Onemethod for eliminating an autoclave process is to utilize a relativelyslow room temperature curing process. However, a better method would beto eliminate the need to process in an autoclave environment whilemaintaining the cure characteristics of an elevated temperature.

All thermoset processes involve heating the thermoset resin orresin-fiber materials, and the extent of heating a mold or autoclave aredictated by the chemistry of the resin. Since processing in an autoclaveenvironment increases the cycle cost, resins have been developed forroom temperature curing. However, slow room temperature cure resins leadto reductions in facility productivity, and thus a large space and alarge number of molds are required to increase production.

Room temperature cures permit the use of low cost mold materials such aswood, plaster or syntactic foam. However, when consideration of costs islimited to mold costs only, the total effect on the cost of themanufacturing process is lost. Fabrication costs for room temperaturecure parts are also driven by the time and space required. Total costsinclude cost for tooling, molds and productivity costs to produce afinal product. When the hours of time and large space used to massproduce a room temperature cured product is considered, the advantage oflow tooling and mold cost is lost. To increase production, a largenumber of molds are required and space and mold costs then make theprocess expensive. As a result, volume mass production costs for theroom temperature cure process are higher.

The heated platen thermoset cure process is the fastest cure now used.This process is well suited for mass production of parts, but the moldmust be loaded quickly since the cure starts on contact with the hotmold. Thus large parts or parts requiring time consuming lay ups cannotbe used with this process. Furthermore, loading of the heated metalplaten press mold is difficult since the molds are hot, and must beoperated continually hot in order to shorten cycle times. These metalmolds are also expensive.

As indicated above, autoclaves are expensive, and cures withinautoclaves are relatively slow since the entire autoclave and mold mustbe heated and cooled in the production cycle. In addition, oven orautoclave cures, use higher cost mold materials such as metals and fiberreinforced plastics (FRP). Also, molds for oven, autoclave or heatedplaten press processes often utilize imbedded heating or coolingelements to control the process temperatures. Manufacturing costs for athermally controlled mold or a heated platen press mold are high, andthe time required to produce these molds may range from weeks to months.Thus total costs incurred must include mold fabrication and processproductivity costs. While heated and cooled molds result in lowerproduction costs due to increased productivity, these costs are offsetby substantially higher mold costs.

SUMMARY OF THE INVENTION

Since the manufacturing costs of plastic parts are influenced by theresin, curing process, tooling, machinery and production space required,the transparent mold system (TMS) of the present invention provides fora significant reduction in manufacturing costs by an integrated processthat reduces the cost of each item. The transparent mold and visiblelight heating process of the invention yields an integrated process thataddresses all requisite items to make thermoset resins and thermoset FRPsuitable for economical mass production.

The transparent mold and visible light heat curing process (TMS) of theinvention reduces the manufacturing costs for producing thermoset resincomposites, and contrasts with the existing cure systems which aresubstantially more expensive and slower. The invention also eliminatesthe need to process molds and parts in an autoclave and utilizes lowcost transparent molds and a controlled visible light energy heat sourceto elevate rapidly the thermoset resin to the cure temperature.

The integrated manufacturing process of the invention exploits theoptical properties of polycarbonate and other transparent mold materialsand utilizes discrete, light energy heat sources in the visible range tomold and heat cure thermoset resins and thermoset FRP resins. The use oftransparent materials for the molds allows the use of visible light asthe heating energy source for the curing process without significantlyheating the mold or any other component of the system. For example, apolycarbonate mold transmits light in the wavelengths between 450 and900 nm (nanometers) and is essentially opaque to radiation at longer andshorter wavelengths. Other transparent mold materials have slightlydifferent transmittance spectrums. The visible light heat radiation isabsorbed by the black or other optimum color of the uncured thermosetresin which quickly heats to an elevated cure temperature. The thermosetpart is rapidly cured without the need to heat an autoclave, platen,oven or mold.

The visible light curing process of the invention also has anothersignificant advantage over room temperature hot platen andautoclave/oven processes. It is possible to elevate temperature quicklyand hold a median temperature at the wet-out temperature of the uncuredFRP thermoset resin. The wet out temperature is the temperature wherethe resin liquefies and has its lowest viscosity so that the resin flowseasily throughout the transparent mold and FRP matrix. Fully wetting thematrix increases the probability of manufacturing an acceptablecomposite part. After wetting out, the temperature is then almostinstantly raised to the elevated cure temperature for a rapid cure. Itis not practical to control precisely the dwell time in the wetting outtemperature range with either room temperature, hot platen orautoclave/oven cures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates diagramatically the light curing apparatus andprocess of the invention wherein a light source in the visible spectrumis used to radiate thermal energy into an uncured thermoset resinenclosed within a transparent mold;

FIG. 2 is a chart which compares the molding process of the invention(TMS) with other known processes relative to a mass production costfactor; and

FIG. 3 is a graph showing the percent of energy transmittance forvisible light through a clear polycarbonate mold in comparison withultraviolet light and infrared radiation through the mold.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An article or part P is molded of a thermoset plastic resin or fabricsor fibers impregnated or wetted with a thermoset plastic resin. Thethermoset resin is formulated to absorb heat energy in the visible lightspectrum by the selection of the color of fabric or resin or the colorof both, and by selecting the wavelength of the light energy.

A visible light transparent mold 10 (FIG. 1) is generally illustrated bya set of arcuate walls 12 and 14 separated by resilient seals 16 and thewalls are held together by a suction within a tube 17 to define a moldcavity 18. The walls are constructed of a material that transmits aselected radiated energy in the visible spectrum. The mold walls 12 and14 are optimized to transmit energy to the resin inside the mold whileabsorbing a minimum amount of energy in the transparent mold material.This increases heating efficiency of the resin and maintains low moldtemperatures needed for transparent plastic molds. Various glass andplastic transparent molds are suitable for a range of temperatures. Highenergy efficiency is possible since the mold 10 is not heated andcooled, and only the thermoset resin part P is heated.

A hybrid mold system may be used. For example, a metal or otherconventional opaque molding material may be used to form the wall 14while the other wall 12 is transparent. The transparent wall 12 may beused to preassemble a complex time consuming cold lay up of FRP. Theconventional opaque wall 14 is kept continually hot with internalheating to assist a cure or just cause the resin to wet out. Radiatedvisible light energy (heat) is applied through the transparent wall 12of the mold to accelerate curing of the resin after the mold is closed.

Transparent molds can be made hollow to carry cooling gas or fluid formore flexibility in adjusting mold temperatures. This option isdesirable in thicker parts. Cooling is also desirable with resins thatexotherm excessively. Multiple segments of transparent molds may be usedfor forming undercut sections in the resin part or to demold partswithout draft. A slightly flexible transparent plastic mold allows formold twisting to facilitate part demolding in the same fashion as aplastic ice cube tray functions. Flexible transparent molds also allowfor demolding using a peeling action. Either wall or both transparentwalls of the mold may also be incorporated into the finished part andbecome a transparent surface of a finished composite structure part bynot using a parting agent or by incorporating a bonding agent on theinner surface of each transparent mold wall.

Heat sensors (not shown) may be laminated into the resin part P orenclosed in the transparent mold 10, or non-contact temperature sensingequipment 19 may be used to read the surface temperature of the part Pthrough the transparent mold. The sensing equipment 19 scans the curingpart to identify "cool" spots and directs extra visible light energyfrom spotlights 20 to the cool areas. The energy source lights 20 areselectively turned on and off to control and limit the temperature ofthe resin. The temperature sensing system feeds data to a closed-looptemperature controller 25 for visible light energy control to maximizeresin curing speed and temperatures without exceeding the resintemperature limit.

The controller 25 processes the resin part temperature information fromthe temperature sensors. The energy source lights 20 are turned on andoff or aimed to achieve the desired part temperatures. The closed-loopprocessor or controller 25 may provide for rapid scanning of the part Pin the mold 10. Hot spots will cause an associated light 20 to turn off.Cold spots will command aimable articulated spot lights 20 toconcentrate on a cold area to bring that area temperature up to matchthe rest of the part being cured. Cycle time is minimized by heating theenclosed thermoset resin part P to an isotherm without exceeding theresin maximum cure temperature.

The wavelengths of the visible light heat sources 20 are selected tooptimize energy in the frequency spectrum for part heat absorption andto minimize the heat absorption of the transparent mold 10. The array oflights 20 are controlled by the closed-loop temperature controller 25which blinks lights on and off to adjust the curing temperature of thepart P. Auxiliary articulated spotlights 20 may also be controlled toaim extra visible light energy at cool spots. The objective is tomaintain an even rapid build up of temperature throughout the thermosetpart P as the part cures.

Light filters 30 are used to adjust the spectrum of the light energy.Light energy in the ultraviolet (U.V.) and infrared (I.R.) spectrumswill heat transparent plastic molds. This is undesirable, but thefilters 30 avoid heating the transparent mold 10. For example, lowemissivity glass filters will reflect I.R. energy, and U.V. filterbarrier coatings will eliminate energy in the U.V. spectrum. Theobjective is to maximize radiation of energy in the selected visiblewavelengths which are transmitted through the transparent mold walls toheat cure the resin while eliminating other wavelengths that would heatthe transparent mold walls. All plastics used in the transparent moldshave a heat distortion temperature that should not be exceeded. Thelight filters 30 minimize energy spectrums that are absorbed by thetransparent molds thereby assisting in maintaining the transparent moldsat a low temperature.

The vacuum seals 16 may be any conventional seal material compatiblewith the thermoset resins and transparent mold resins and temperatures.Bleeder blankets that do not interfere with the visible light energytransmission can be used to distribute vacuum within the transparentmold. The number of vacuum ports is selected to remove excess air andresin as long as the plumbing does not block light energy from thetransparent wall of the mold.

Air, water or other transparent cooling fluids or gasses from a bloweror pump 32 may be directed over or through the transparent mold 10 toreduce mold temperatures during the resin heating and curing process. Ifadditional pressure is required to form or shape the part P, the mold 10may be enclosed within a pressure vessel 35. A transparent pressurevessel 35 may have the visible light energy sources or lamps 22 locatedoutside of the vessel 35. Conventional opaque pressure vessels may beused when the light energy sources are located along with thetransparent mold inside the vessel.

A mold release agent may be used with the transparent molding system ofthe invention. Since maximum transmissibility in the selected lightenergy spectrum is desired, chemical compatibility of the release agentwith the transparent mold materials and the thermoset resins is alsodesired.

The transparent mold system of the invention provides for low cost andrapid manufacturing of thermoset composite structures. The system avoidsroom cure, heated platen press and autoclave processes and thecorresponding costs and complexities associated with these processes.The transparent mold production tooling of the invention provides formajor cost reductions and productivity increases. Cost savings areachieved by the speed (a number of minutes) which the transparent moldsare cycled in the manufacturing process and the low cost of producing alarge number or transparent molds required for mass production. Theutilization of transparent molds enables the two to be addressedsynergistically, that is, high cycle rate and low mold cost.

Transparent molds are able to exploit the cost effectiveness of existingconventional thermoset resins, low tooling costs and fast process times.Transparent molds are quickly mass produced from low cost transparentmaterials such as clear polycarbonate. Fabrication costs of transparentmolds therefore are less than other thermoset molds. Thus the visiblelight curing process of the invention results in process times and totalcosts that are substantially less than other thermoset cure processes,as shown in FIG. 2.

The molding process of the invention has two additional areas inproduction tooling that provide for major manufacturing costs andproductivity savings. They are mold replication and quality control.Replication is the manufacture of additional molds to meet productionrequirements. Replication not only involves the cost to replicateadditional tooling exactly, but also the lead time required to producecomplex additional molds. Quality control is the ability to replicateexactly the master mold. Quality is highly dependent upon thereplication process. The molding process of the invention uses onemaster mold to produce a large quantity of replicated molds.

Existing techniques for fabrication of molds for conventional thermosetcure processes takes weeks or months to produce an oven/autoclave/heatedpress cure mold. The fabrication techniques and materials commonly usedinvolve time intensive machining or casting operations to produceindividual molds. Replication of additional molds requires either an upfront commitment to concurrent multi-mold fabrication costs oradditional lead time to fabricate the required number of expensivemolds. In either case, the high fabrication cost of each additional moldremains essentially constant.

Mold quality control is an important factor when a mold design is to bereplicated. Variances in conventional heated and cooled molds eithermust be accepted or high costs will be incurred in assuring thereplicated molds exactly match the master mold. Inexpensive transparentmolds of the present invention are produced by casting, vacuum formingor pressure-forming a clear material over a master mold. Replication ofadditional transparent molds utilize the same master mold. The high moldmachining, casting, and fabrication costs are incurred only once on themaster mold. As a result, a significant cost advantage is realized withthe (TMS) second mold and continues with every additional transparentmold replicated.

The mold production process of the invention is a high production rateprocess and a large number of additional molds can be fabricated withina day as opposed to weeks or months for conventional molds. Since theforming process uses one master mold, all replicated molds areidentical, and strict quality control is inherent in the replicatingprocess. As a result of the ability to replicate molds rapidly, moldsused in the molding process of the invention are able to transitionquickly to production. The ability to transition rapidly to productionincreases facility productivity and provides for making quick productchanges.

While the molding method and form of apparatus herein describedconstitutes a preferred embodiment of the invention, it is to beunderstood that the invention is not limited to the precise method andform of apparatus described, and that changes may be made thereinwithout departing from the scope and spirit of the invention as definedin the appended claims.

The invention having thus been described, the following is claimed:
 1. Amethod of molding an opaque article from a non-photosensitiveheat-curable thermoset plastics resin having a dark heat-absorbingcolor, comprising the steps of inserting the resin within a cavity of amold having a substantially transparent wall portion formed of amaterial substantially transparent to visible light, closing the mold toconfine the non-photosensitive dark color resin within the cavity,heating the resin without substantially heating the transparent moldwall portion by directing visible light energy into the resin throughthe transparent wall portion for absorption by the dark color resin, andcontrolling the amount of the visible light energy directed through thetransparent wall portion according to the temperature required forheat-curing the non-photosensitive resin.
 2. A method as defined inclaim 1 and including the steps of sensing the temperature of thenon-photosensitive resin while the resin is being heated, andcontrolling the amount of visible light energy directed through thetransparent wall portion in response to the temperature sensed.
 3. Amethod as defined in claim 2 wherein the temperature of thenon-photosensitive resin is sensed by a non-contact sensor positioned todetect the resin temperature through the transparent wall portion.
 4. Amethod as defined in claim 1 and including the steps of forming the moldwith opposite wall portions of the material substantially transparent tothe visible light and defining the cavity between the wall portions forconfining the resin, and directing the visible light energy through bothof the opposite transparent wall portions for decreasing the time forheat curing the resin.
 5. A method as defined in claim 4 and includingthe step of forming the mold to hold a vacuum within the cavity betweenthe walls, and creating a partial vacuum within the cavity.
 6. A methodas defined in claim 4 and including the step of pressurizing theopposite wall portions of the mold towards each other with substantiallytransparent fluid pressure for compressing the resin according to theshape of the cavity.
 7. A method as defined in claim 1 and including thestep of cooling the mold by directing a substantially transparentcooling fluid in contact with the mold.
 8. A method as defined in claim1 wherein the visible light energy has a wavelength range generallybetween 450 and 900 nanometers.
 9. A method of molding an opaque articlefrom a non-photosensitive heat-curable thermoset plastics resin having adark heat-absorbing color, comprising the steps of inserting the resinwithin a cavity of a mold having a substantially transparent wallportion formed of a material substantially transparent to visible light,closing the mold to confine the non-photosensitive dark color resinwithin the cavity, heating the resin without substantially heating thetransparent wall portion by directing through the wall portion into theresin visible light energy, sensing the temperature of the dark colorresin while the resin is being heat cured, and controlling the amount ofvisible light energy directed through the transparent wall portion intothe dark color non-photosensitive resin in response to the temperatureof the resin being sensed.
 10. A method as defined in claim 9 whereinthe temperature of the resin is sensed by a non-contact sensorpositioned to detect the resin temperature through the transparent moldwall portion.
 11. A method as defined in claim 9 and including the stepsof forming the mold with opposite wall portions of the materialsubstantially transparent to visible light and defining the cavitybetween the wall portions for confining the resin, and directing thevisible light energy through both of the opposite mold wall portions fordecreasing the time for curing the resin.
 12. A method as defined inclaim 9 and including the step of forming the transparent mold to hold avacuum within the cavity, and creating a partial vacuum within thecavity.
 13. A method as defined in claim 9 and including the step ofpressurizing the outer surfaces of the mold wall with substantiallytransparent fluid pressure for compressing the resin according to theshape of the mold cavity.
 14. A method as defined in claim 9 andincluding the step of cooling the mold by directing a transparentcooling fluid over the mold walls.